Optical observations of Be/X-ray transient system KS 1947+300

ROTSE-IIId observations of the Be/X-ray transient system KS 1947+300 obtained between September 2004 and December 2005 make it possible to study the correlation between optical and X-ray activity. The optical outburst of 0.1 mag was accompanied by an increase in X-ray flux in 2004 observations. Strong correlation between the optical and X-ray light curves suggests that neutron star directly accretes from the outflowing material of Be star. The nearly zero time lag between X-ray and optical light curves suggests a heating of the disk of Be star by X-rays. No optical brightening and X-ray enhancement was seen in 2005 observations. There is no indication of the orbital modulation in the optical light curve

The Spectrum and Dips of RE 0751+14: A joint evaluation of ROSAT and ASCA Archival Data

Using archival ASCA and ROSAT observations of RE 0751+14, X-ray energy spectra, pulse profiles and the results of pulse timing analysis are presented. The energy spectra are well-fitted by a blackbody model at low energy and a Raymond-Smith model at high energy, together with a partial covering absorber. A fluorescence emission line at 6.4 keV with an equivalent width $\sim 220$ eV was resolved for the first time.Comment: To appear on Astrophysics and Space Science, vol 259, pages 191-203, January 199

AB Dor: A Single Star with RSCVn Like Activity in X-ray Band

Using the archival ROSAT PSPC observations, AB Dor is found to be variable in X-rays. The periodic variations are consistent with previously reported rotational period of 0.514 days. The average spectrum of AB Dor is best represented with two-temperature Raymond-Smith model with kT values of 0.19+/-0.07 and 1.17+/-0.02 keV. The quiescent luminosity of the system is found to be 4.36+/-0.6 10^30 ergs/s. A flare with a rise time of about 350 seconds is detected during which X-ray luminosity rises from 5.8+/-1.6 10^30 to 15.8+/-4.9 10^30 ergs/s. We conclude that AB Dor is very similar to the active components of RS CVn binaries and other active classes. In view of the wide separation from the binary companion Rst 137B, this activity must be intrinsic to the active star.Comment: 11 pages in PostScript including figures, accepted for publication in Astrophysics and Space Science, available at http://astroa.physics.metu.edu.tr/preprints.htm

ROSAT Observations of the Vela Pulsar

The ROSAT HRI was used to monitor X-ray emission from the Vela Pulsar. Six observations span 2-1/2 years and 3 glitches. The summed data yield a determination of the pulse shape, and X-ray emission from the pulsar is found to be 12 % pulsed with one broad and two narrow peaks. One observation occurred 15 days after a large glitch. No change in pulse structure was observed and any change in X-ray luminosity, if present, was less than 3 %. Implications for neutron star structure are discussed.Comment: To be publisned in the Astrophysical Journa

First hours of the GRB 030329 optical afterglow

We present the first results of the observations of the extremely bright optical afterglow of gamma-ray burst (GRB) 030329 with the 1.5m Russian-Turkish telescope RTT150 (TUBITAK National Observatory, Bakyrlytepe, Turkey). RTT150 was one of the first 1.5m-class telescopes pointed to the afterglow. Observations were started approximately 6 hours after the burst. During the first 5 hours of our observations the afterglow faded exactly as a power law with index -1.19+-0.01 in each of the BVRI Bessel filters. After that, in all BVRI filters simultaneously we observe a steepening of the power law light curve. The power law decay index smoothly approaches the value ~= -1.9, observed by other observatories later. This power law break occurs at t-t_0 =0.57 days and lasts for +-0.1 days. We observe no variability above the gradual fading with the upper limits 10--1% on time scales 0.1--1000s. Spectral flux distribution in four BVRI filters corresponds to the power law spectrum with spectral index \alpha=0.66+-0.01. The change of the power law decay index in the end of our observations can be interpreted as a signature of collimated ultrarelativistic jet. The afterglow flux distribution in radio, optical and x-rays is consistent with synchrotron spectrum. We continue our observations of this unique object with RTT150.Comment: Astronomy Letters, Vol. 29, No. 9, p. 573; 6 pages, 5 figures; pagination corrected; the original Russian version can be found at http://hea.iki.rssi.ru/~br/030329/pfh030329.pd